1. Field of the Invention
This invention relates to machine tools and more particularly to machine tools having means for holding a first and a second workpiece.
2. Description of the Prior Art
The prior art knows many types of machines for holding a first and a second workpiece. The basic advantage of this type of machine is an increase in the percentage of actual machining time and a decrease in the percentage of workpiece installation or set-up time over a single workpiece machine. The plural workpiece machines generally have a first and a second workpiece mounting unit each of which is movable between a standby position and a machining position. While one workpiece, for example, the first workpiece is being machined on the first workpiece mounting unit, a second workpiece can be mounted on the second workpiece mounting unit which is located in the standby position. After machining of the first workpiece, the first workpiece is shifted to the standby position and the second workpiece is shifted to the machining position. The first workpiece can them be removed and replaced by a third workpiece on the first mounting unit during machining of the second workpiece.
The plural workpiece principle is incorporated in various types of machines one of which includes a single horizontal slide having a first and a second mounting unit on the ends of the slide. While the first workpiece on the first mounting unit is being machined the second workpiece can be installed on the second mounting unit in the standby position. Upon completion of machining of the first workpiece, the first mounting unit is moved to the standby position enabling machining of the second workpiece. The first workpiece can then be removed allowing installation of a third workpiece. The basic disadvantage of this machine is that the mounting unit in the standby position moves in accordance with the movement of the mounting unit in the machining position.
Others in the prior art use a pallet shuttle system, for example, U.S. Pat. No. 3,099,873. The pallet shuttle system includes a first and a second pallet having first and second mounting units. The machine has a standby position and a machining position. The pallets are loaded with workpieces in the standby position and are individually moved to the machining position for machining by a workpiece tool. Tapered pins on the upper side of a slide in the machining position engage with tapered holes on the bottom side of the pallet to locate the pallet in the machining position. Upon completion of the machining of the first workpiece on the first pallet, the pallet is quickly moved to the standby position and the second pallet is moved to engage with the tapered pins of the slide in the machining position to enable machining of a second workpiece.
The pallet shuttle system operates satisfactorily but is inaccurate due to errors resulting from locating the pallets on the slide in the machining position. Those skilled in the are realize that tolerance errors are accumulative and manufacturers of high quality machine tools strive to minimize accumulative error to maintain the quality and standard of their machine tools. In the pallet shuttle system, a first error is introduced upon placing the workpiece on the pallet. A second error is introduced upon locating the pallet on the slide in the machining position through the tapered pins. Finally, a third error is introduced in the machanism for moving the tool relative to the workpiece. Accordingly, three sources of error accumulate in each direction in the pallet shuttle system.
Therefore, an object of this invention is to overcome the aforementioned disadvantages of the plural mounting units on a single slide and the pallet shuttle system by providing a machine tool capable of rapid machining of plural workpieces with higher accuracy then heretofore possible.
Another object of this invention is to provide a machine tool with plural slides which has only two accumulative errors in each direction rather than three accumulative errors found in the pallet shuttle system.
Another object of this invention is to provide a machine tool with plural slides for machining a first workpiece in a machining position with a second workpiece in a standby position and for interchanging the status of the first and second workpieces without the use of a pallet shuttle system.
Another object of this invention is to provide a machine tool with plural slides wherein each slide has a table which is rotatable about an axis which is perpendicular to the direction of movement of the slides.
Another object of this invention is to provide a machine tool with plural slides having novel control means for controlling the machining of the first and second workpieces through a single control system.
Another object of this invention is to provide a machine tool with plural slides which is inherently more accurate than the pallet shuttle system.